Just in my personal opinion, and I'm gonna do it based on the category they were first in when they competed in the series:

Boys
James Arthur
Joe McElderry
Shane Ward
Matt Cardle

Girls (hardest category, I'd love to include Laura, Diana, Lauren, Amelia and Misha too)
Leona Lewis
Alexandra Burke
Stacey Solomon
Ella Henderson

Groups
Little Mix
JLS
Only The Young
Blonde Electra

Overs
Olly Murs
Ruth Lorenzo
Kitty Brucknell
Fleur East